1. Field of the Invention
The invention relates to a nickel-base superalloy, in particular for the fabrication of monocrystalline components.
2. Discussion of Background
Nickel-base superalloys of this type are known. These alloys are used to fabricate monocrystalline components, in particular turbine blades of gas turbines. By means of these monocrystalline components, the mechanical strength of the material at high temperatures can be maximized. As a result, the inlet temperature of gas turbines can be increased, with a consequent rise in the efficiency of the gas turbine.
Monocrystalline alloys as disclosed by U.S. Pat. No. 4,643,782, Swiss Patent No. 637 165, EP 0 076 360, U.S. Pat. No. 5,270,123 and EP 0 208 645 to this end include solid solution strengthening elements such as Re, W, Mo, Co, Cr, as well as elements forming .gamma.' phases such as Al, Ta and Ti. The percentage of refractory alloy elements in the base matrix Ni increases continually with the increase of the operating temperature required for the alloys. Customarily, a monocrystalline alloy contains (in wt %): 6-8% of W, up to 6% of Re and up to 2% of Mo. The alloys described in the abovementioned publications exhibit high creep resistance, good LCF (low cycle fatigue) and HCF (high cycle fatigue) characteristics, as well as high oxidation resistance.
These known alloys were developed for aircraft turbines and therefore optimized with respect to short-term and medium-term characteristics, i.e. the service life is designed to be up to 20,000 hours. In contrast, industrial gas turbine components have to be designed for a service life of up to 75,000 hours. By virtue of the optimization in terms of maximum creep strength at high temperatures and high stresses, these alloys have a positive heat treatment window i.e. the primary .gamma.' phase is completely dissolved without incipient melting of the alloy.
Thus, an alloy "CMSX-4" from U.S. Pat. No. 4,643,782, when used in industrial gas turbines, does indeed exhibit good creep resistance, but has a high susceptibility to the formation of small-angle grain boundaries, of freckles, i.e. imperfections due to a chain of equidirectional grains having a high eutectic content, and also to extensive recrystallization of the monocrystalline components during solutionizing After an operating time of 3000 hours at temperatures above 1000.degree. C., the alloy shows marked coarsening of the .gamma.' structure, followed by an increase in the creep rate.
Swiss Patent No. 637 165, EP 0 208 645 and "Second generation nickel-base superalloy", A. D. Cetel et al., Superalloys 1988, ed S. Reichman et al., Met. Soc., 1988, p. 235, disclose alloys designated as "PWA", in particular "PWA 1484", and EP 0 076 360 and U.S. Pat. No. 5,270,123 disclose alloys designated as "Ren", in particular "Rene N5", but these have drawbacks similar to those of the alloy CMSX-4.
By means of the alloying elements chosen, a positive or negative lattice offset between .gamma. phase and .gamma.', phase is generated in the abovementioned alloys. As a result of the lattice distortion, .gamma.' grains impede slippage or intersection of dislocations. While said lattice distortion does bring about an increase in the low-cycle fatigue strength, it also causes, with prolonged loading, coarsening of the microstructure and, correspondingly, a long-term mechanical weakening of the alloy.